Reactor for isoparaffin olefin alkylation

ABSTRACT

A reactor for the autorefrigerant alkylation process has a reactor vessel with a lower end inlet for the refrigerant reactant and the sulfuric acid and a series of inlets for the olefin reactant at vertically spaced intervals. A flow path for the reactants is provided by co-acting baffles which define sequential reaction zones. The baffles interact with a rotary mixer with multiple impellers to provide agitation. Outlets for the vaporized refrigerant and the reaction effluent are provided at the upper end of the vessel. In the alkylation process, the liquid isoparaffin hydrocarbon reactant/refrigerant with a sulfuric acid alkylation catalyst is introduced into the lower end and passed along the extended reactant flow path with the olefin reactant introduced at intervals along the path. The reaction mixture flows in the sequence of serial reaction zones within the reactor to promote mixing of the isoparaffin reactant with the acid catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/380,098, filed on Feb. 24, 2009, which claims priority fromU.S. application Ser. No. 61/071,773, filed 16 May 2008.

FIELD OF THE INVENTION

This invention relates to a reactor which is useful in theisoparaffin-olefin alkylation reaction and, more particularly, to areactor which is useful for the sulfuric acid catalyzedisoparaffin-olefin alkylation process.

BACKGROUND OF THE INVENTION

One of the common petroleum refinery processes for the upgrading oflight ends to high octane gasoline is the isoparaffin-olefin alkylationprocess in which an isoparaffin, usually isobutene is alkylated with alight olefin, usually propylene, butene or mixtures of the two, toproduce a high octane liquid product in the gasoline boiling range. Thealkylate product is considered a premium gasoline blending component dueto its high octane, low RVP, low sulfur content and low distillation T90point. Two major isoparaffin-olefin alkylation process have becomewidely accepted in the refining industry, the sulfuric acid process andthe hydrofluoric acid (HF) process which, while fundamentally similar,possess different characteristics arising from the different abilitiesof the two acids to catalyze the alkylation reaction. These twoprocesses are by now well established in the refining industry and eachis recognized as having its own technical and economic advantages andproblems.

Current worldwide gasoline demand, along with more stringentenvironmental limitations, are driving refineries to expand and considernew alkylation units. While the process economics may in many casesfavor the HF process, economics alone may not always be the determiningfactor and sulfuric acid alkylation has retained a major portion of thealkylation capacity in the industry. Many of the new projects are likelyto choose sulfuric acid as catalyst as a result of perceptionsconcerning the safety and environmental concerns about HF alkylation inspite of its excellent plant safety and environmental record. Solid acidcatalyst technology, while attractive in principle, is, however, quitefar from being sufficiently well established for widespread commercialacceptance, leaving sulfuric acid as a currently viable option. in spiteof reactor reliability issues, especially with rotating equipment andinternals.

There are two major variants of the sulfuric acid alkylation processwhich differ principally in the means used to remove the heat of thealkylation reaction. The DuPont™ Stratco™ process, the dominant processalso known as the effluent refrigeration process, uses a liquid fullreactor/acid settler system in which the heat of reaction is removed byan internal tube bundle, making the reactor resemble a shell-and-tubetype heat exchanger although with an agitator which is used to securegood contact between the acid and the hydrocarbon reactants. Control ofthe pressure in the flash drum maintains the contents of the reactor inthe liquid phase at by appropriate control, the temperature of thereactant mixture is kept at a desired value. The autorefrigerant processpioneered by Esso (now ExxonMobil), by contrast, uses a reactor in whichthe heat of reaction is removed by operation at a pressure at which aportion of the hydrocarbon charge boils. The reactor used in theautorefrigerant process conventionally comprises a single horizontalvessel divided into mixing chambers with one to two separate pressurezones. Plug flow is achieved by cascade operation with the refrigerantand sulfuric acid being admitted at one end of the contactor and theolefin being introduced progressively in the mixing chambers by means ofspargers with vigorous mixing provided by driven impellers or, incertain cases, eductor type mixers. The reactant mixture and acidcatalyst passes from chamber to chamber over weirs and through apressure let down between the two pressure zones. Vaporization of therefrigerant removes the heat of reaction and very low temperatures canbe achieved while operating at low pressure.

Considering the available options for sulfuric acid alkylation, the maintechnical disadvantages of the Stratco contactor can be summarized asthe following: liquid phase operation requires higher pressure thanautorefrigerated reactors and therefore it operates at a highertemperature which promotes secondary reactions; the hydrodynamics in thecontactor consist of high velocity pumping of liquid by the single mixerrequired to create high liquid velocities over the heat exchanger tubes.As a result, the emulsion is subjected to very high energy dissipationlocally in the region of the mixer blades and low energy dissipationeverywhere else; the compressor energy requirement is between 15 and 20%higher than with autorefrigerated systems; the capacity of a contactoris typically 1200 to 2000 BPD (about 1900-3200 hl/day) of alkylate.Therefore, a typical unit will require between 5 and 8 reactors; andeach reactor needs its own settler, pumps, and control system.

With the autorefrigerant process, most units require only one reactorand settler. However, there are some disadvantages: Vessels can get verylarge: 12 -15 ft (about 3.6-4.5 m.) diameter and over 100 ft (about 30m.) in length; long residence times affect alkylate quality due toalkylate decomposition reactions; multiple mixing zones require multiplemixers, motors, and gearboxes which has an effect on maintenance andreliability control resources; the horizontal configuration isconsidered less efficient in terms of volume utilization; and the largesizes make it less competitive for units smaller than 10 kBD (about15900 hl/Day) whereas many refineries will normally be satisfied by acapacity of 5-10 kBD (about 8000-16000 hl/day).

Given the continued viability of the autorefrigerant sulfuric acidalkylation process, it would be desirable to incorporate improvementswhich negate or offset at least some of the disadvantages mentionedabove.

The present invention is concerned with improvements to theautorefrigerant alkylation process. Accordingly, in this specification,the term “alkylation” is used to refer to the isoparaffin-olefinalkylation process of the petroleum refining industry in which a lightolefin (C₂-C₆, usually C₃-C₄) is used to alkylate a light isoparaffin(C₄-C₆, usually isobutane) to produce a liquid alkylation product whichis predominantly in the gasoline boiling range. The autorefrigerantalkylation process, referred to as such in this specification is thealkylation process in which heat of the alkylation reaction is removedby vaporization of a reactant hydrocarbon refrigerant. Exemplary patentsdescribing variants of the autorefrigeration alkylation process includeU.S. Pat. No. 2,429,205 (Jenny); U.S. Pat. No. 2,768,987 (Hart); U.S.Pat. No. 2,903,344 (Rollman); U.S. Pat. No. 3,170,002 (Kelso); U.S. Pat.No. 2,852,581 (Stiles); U.S. Pat. No. 2,859,259 (Stiles) and U.S. Pat.No. 2,920,124 (Stiles).

SUMMARY OF THE INVENTION

According to the present invention, an improved reactor is provided forthe autorefrigerant alkylation process. The reactor, which operates inthe same fundamental manner as the conventional autorefrigerated reactorwith three phases, acid liquid, hydrocarbon liquid and hydrocarbonvapor, is characterized by a upright, closed, generally cylindricalreactor vessel disposed with an inlet or inlets for the refrigerantreactant and the sulfuric acid at its lower end and a series of inletsfor the olefin reactant at vertically spaced intervals up the length ofthe reactor. An extended flow path for the reactants is provided bymeans co-acting baffles which define sequential reaction zones in whichalkylation takes place with the reaction mixture of isoparaffin, olefinand catalyst following an extended, sinuous or serpentine flow path asit ascends the reactor. The baffles interact with a rotary mixer withmultiple impellers located on the reactor axis which provides agitationto the mixture ascending the reactor additional to that created by theebullition of the refrigerant. An outlet or outlets for the vaporizedrefrigerant and the reaction effluent are provided at the upper end ofthe vessel.

Notable advantages of the reactor and of the autorefrigerant processoperate in the improved reactor are: (i) the vertical vessel design ismore consistent with current reaction engineering technology; (ii)efficient volume utilization ensures a fast alkylation reaction whilelimiting unwanted secondary reactions; (iii) relatively low residencetime avoids alkylate degradation; (iv) a single driver motor on themixer simplifies rotary equipment design, cost and maintenance,especially with respect to the seals which, being in the vapor space areless subject to erosion and failure than in the liquid phase Stratcoprocess; and (v) the reactor can be designed with capacity within thecurrent technology range using one vessel.

The process according to the invention comprises introducing a liquidisoparaffin hydrocarbon reactant/refrigerant with a sulfuric acidalkylation catalyst into the lower end of a generally cylindricalreactor arranged with a substantially vertical longitudinal axis. Thereactor also has a sequence of serial reaction zones defined by aplurality of co-acting baffles at vertically spaced intervals thatprovide an extended reactant flow path in the reactor with the reactantsflowing in a sinuous path as they ascend the reactor which promotesvigorous mixing of the reaction mixture as it passes through and up thereactor. Olefin reactant is introduced into the reactant flow path ineach of the sequential reaction zones to react with the isoparaffin inthe alkylation reaction. With the evolution of the heat of reaction, aportion of the refrigerant reactant is vaporized to effect temperaturecontrol in the reactor. Additional mixing and agitation is provided bymeans of the rotary mixer with its mixing impellers in each of thesequential reaction zones. The vaporized reactant refrigerant andalkylation reaction products leave the reactor at its upper end with thevaporized refrigerant passing to the refrigeration compressor forrecycle to the reactor and the alkylation reaction products to theproduct recovery section of the unit.

DRAWINGS

In the accompanying drawings: FIG. 1 is a simplified vertical crosssection of an autorefrigerant alkylation reactor, and

FIGS. 2A and 2B show the configurations of the two types of baffles inthe reactor of FIG. 1.

DETAILED DESCRIPTION

The alkylation reactor shown in FIG. 1 has six reaction zones but thereare no theoretical limitations on the number of zones that can beprovided. The final number of zones will depend on the vessel practicalsize, hydrodynamic limitations such as coalescing, as well as mixerdesign but in most cases from four to ten zones will be adequate andconvenient. The reactor comprises a vessel 10, cylindrical in form withits central longitudinal axis disposed vertically; the vessel has sidewall portion 11 closed at each end by curved lower end plate 12 and asimilar upper end plate 13, defining a closed reaction space. An inletport 15 for refrigerant recycled from the refrigeration system andsulfuric acid alkylation catalyst recycled from the acid settler isprovided at the bottom of the vessel located on the axis of the vessel.The refrigerant is fed to inlet port 15 by way of line 16 with thesulfuric acid catalyst entering through line 17, the two being mixedtogether before passing into the vessel though port 15. The olefinreactant premixed with additional isoparaffin is introduced into thevessel by means of a series (six in number, one in each reaction zone)of spargers 20 supported on feed lines 21 which enter through the sidewall 11 of the vessel and deliver the reactants in a uniform pattern ineach reaction zone (for clarity only the lowest feed line 21 and itsconnected sparger 20 are indicated in the lowest reaction zone 35 a; asimilar arrangement is used in each successive reaction zone 35 b-35 fin the reactor). The individual feed lines to each sparger 20 receiveincoming reactant by way of line 22. Longitudinal anti-swirl baffles 24(two shown), which extend vertically along the inside of side wall 11 ofthe vessel, are added to assist mixing. Four vertical anti-swirl baffleswould be adequate.

The isoparaffin comprises one of the feed components for the alkylationreaction and also acts as an autorefrigerant for the process, carryingoff heat of reaction by evaporation. Flash zone 25 is located at thebottom of the vessel adjacent inlet 15 and in this zone a portion of therefrigerant isoparaffin evaporates to provide some initial cooling forthe charge. The evaporated isoparaffin vapor passes up through theliquid phase in the reactor and besides providing cooling, also agitatesthe charge in its progress up the vessel. A mixer shaft 26 extends downalong the central axis of the vessel, driven by means of motor 27 andjournalled in seal/bearing 28. Outlets 29 for the liquid phase reactionmixture are provided proximate the top of vessel 10 in the side wallportion 11 of the vessel below upper end cap 13. Vapor outlets 30 inupper end plate 13 provide a means of egress for vapor, mainlyisoparaffin.

The consecutive reaction zones 35 a-35 f are defined by a series ofco-acting horizontal baffles located at vertically spaced intervalsalong the length of the reactor vessel. A primary baffle 36, is mountedin the reactor above flash zone 18 and below the foot of mixer shaft 20on the vessel axis. This baffle comprises a flat plate having thegeneral configuration shown in FIG. 2A which is fixed to the walls ofthe reactor at its opposed ends, extending between the walls of thereactor to define two peripheral flow passages 37 a, 37 b through whichthe mixed acid/refrigerant mixture can pass. The chordal location of theplate edges is selected to provide flow passages adequate to the fluidflow in the reactor. In FIG. 2A, the baffle is shown with a centralaperture 36 c which is not present in the primary baffle at the bottomof the reaction zones but is present in the similar baffles placedhigher up the reactor where these baffles are mounted on the mixershaft. The horizontal baffles which define the consecutive reactionzones are arranged to provide alternating central and peripheral flowpassages for the ascending flow of reactants and refrigerant in thereactor. Baffles 38 have the configuration shown in FIG. 2B with eachbaffle comprising a pair of plate segments 38 a and 38 b extendinghorizontally at the reactor walls and attached to the walls, with acentral flow passage 39 c defined between the two parallel, chordaledges of the plate segments. As mentioned above, baffles 36 whichalternate with baffles 38 have the same general configuration shown inFIG. 2A for the primary baffle plate but also possess a central aperture36 c (not present primary baffle 36) located on the axis of the reactorthrough which mixer shaft 20 extends; each of these baffles is mountedon the mixer shaft. Thus, each of the six reaction zones 27 a-27 f inthe vessel is defined between successive pairs of baffles in the upwardsequence: 36-38-36-38-36-38-36 with the extended, sinuous flow path forthe reactant/refrigerant mixture defined between the baffles and by theperipheral flow passages 37 a, 37 b and central flow passages 39 cdefined by the baffles alternately along the length of the reactorvessel so that the reaction mixture passes alternately outwards towardsthe peripheral flow passages and inwardly towards the central flowpassages. The alignments of the peripheral segmental flow passages 37 a,37 b and the central flow passages 39 c may be angularly shiftedrelative to one another to promote mixing, for example, with the flowpassages in the primary baffle located on a 0°-360° alignment and theflow passages in the successive 36 baffles on alignments of 180°-270°,0°-360°, 180°-270°. Similarly, the central flow passages in baffles 38may be set on alignments of 0°-360°, 180°-270° and 0°-360°. Otherangular displacements may be also used. The cross-sectional areas of theflow passages, both central and peripheral, should be sufficient toallow the desired flow rates up the reactor, making due allowance forthe vapor generated by the evaporation of the refrigerant.

Additional mixing of the reactants and catalyst is provided by means ofa series of six impellers in the form of paddle blades 40 which areattached to mixer shaft 20 at successive vertically spaced intervalswith one paddle blade in each reaction zone between the successivebaffle pairs (for clarity only one paddle mixer is designated in thetopmost reaction zone 35 f, the paddle mixers in the other zones beingidentical). The longitudinal anti-swirl baffles 24 on the inside of sidewall 11 of the vessel co-act with the impellers to assist in the mixing.

As the reaction mixture (isoparaffin/acid catalyst mix initially andthen with added olefin/isoparaffin from the spargers) passes upwardlythrough the reactor vessel, the mixture pursues an extended plug flowpath alternately inwards towards the mixer shaft and then outwards awayfrom the shaft towards the vessel walls. In this way, a longer reactiontime is provided and good mixing is provided at the points where theolefin reactant is introduced.

The reactants will be the same as those conventionally used in theautorefrigerant process, the light olefin being a C₂-C₆ olefin, usuallypropylene or butene and the light isoparaffin a C₄-C₆ isoparaffin,usually isobutene. The liquid alkylation product will comprisesbranch-chain paraffins predominantly in the gasoline boiling range,providing a highly suitable gasoline blend component for the refinery.Reaction conditions (temperature, pressure, reactant ratio) will becomparable to those used in the autorefrigerant process.

1. An isoparaffin/olefin alkylation process which comprises: introducinga liquid isoparaffin hydrocarbon reactant/refrigerant into a lower endof a generally cylindrical reactor vessel arranged with a substantiallyvertical longitudinal axis, introducing a sulfuric acid alkylationcatalyst into the lower end of a generally cylindrical reactor vessel,introducing an olefin reactant in a series of successive reaction zonesin the reactor vessel to form a reaction mixture of isoparaffin, olefinand sulfuric acid catalyst, mixing the olefin reactant with theisoparaffin and sulfuric acid catalyst to form a reaction mixture ofolefin reactant, isoparaffin reactant and sulfuric acid catalyst,passing the reaction mixture along an extended flow path passing througheach of a plurality of sequential reaction zones defined by co-actingbaffles arranged alternately along the length of the reactor vessel todefine an extended, sinuous flow path for the reaction mixture, to reactthe isoparaffin reactant with the olefin reactant in an alkylationreaction, vaporizing a portion of the isoparaffin to effect temperaturecontrol in the reactor, removing vaporized isoparaffin reactantrefrigerant and alkylation reaction products at the upper end of thereactor vessel, passing the reaction mixture of isoparaffin, olefin andsulfuric acid catalyst, and introducing an olefin reactant at successivevertically spaced locations along the length of the reactor vessel.
 2. Aprocess according to claim 1, wherein the series of co-acting bafflesdefine peripheral and central flow passages for the reaction mixturealternately along the length of the reactor vessel.
 3. A processaccording to claim 2, wherein the baffles defining peripheral flowpassages for the reaction mixture each comprise a horizontal plateextending horizontally between the walls of the reactor vessel havingchordal edges defining peripheral flow passages between the chordaledges and the walls of the reactor vessel.
 4. A reactor according toclaim 2, wherein the baffles having the central flow passages eachcomprise a plurality of plate segments extending horizontally at thewalls of the reactor with a central flow passage defined by the chordaledges of the plate segments.
 5. A reactor according to claim 4, whereinthe baffles having the central flow passages each comprise two platesegments extending horizontally at the walls of the reactor with acentral flow passage defined by parallel chordal edges of the platesegments.
 6. A process according to claim 3, wherein the olefin reactantis introduced into the reactor vessel through spargers.
 7. A processaccording to claim 3, wherein a portion of the isoparaffin reactant isflashed into vapor in a flash zone located at the bottom of the reactorvessel above the lower end of the reactor vessel above the point atwhich the isoparaffin reactant and sulfuric acid catalyst areintroduced.
 8. A process according to claim 3, wherein the inlets forrefrigerant isoparaffin reactant and sulfuric acid catalyst enter thereactor vessel at one point through a combined inlet port.
 9. A processaccording to claim 1, wherein the isoparaffin comprises isobutane andthe olefin comprises propylene or butene.